1. Field of the Invention
The present invention relates to an image forming method of a thermal transfer printer, and particularly to an image forming method of a thermal transfer printer in which a color image having multistage gradations is recorded by using resin-based inks.
2. Description of the Prior Art
In general, in a thermal transfer printer, a paper is supported in front of a platen, and a thermal head having a plurality of heating elements formed thereon is mounted on a carriage. Under the condition that an ink ribbon and the paper are sandwiched between the thermal head and the platen, the thermal head is reciprocated along the platen together with the carriage to supply the ink ribbon. The heating elements of the thermal head are selectively energized based on recording information to partly transfer inks of the ink ribbon onto a paper, thereby resulting in an image such as a desired character being recorded on the paper. Such thermal transfer printer is considerably widespread as an output apparatus for a computer, a word processor or the like because it is high in recording quality, low in noise, low in cost and is easy in maintenance.
As a conventional thermal transfer printer, there is known a thermal transfer printer in which an ink ribbon having a wax-based ink with fusion property coated on base such as a plastic film is used to record an image on a paper. Then, when an image having multistage gradations is recorded by using such wax-based ink, there is used a dither method or the like. Recently, in accordance with an increasing demand of a color image having a higher resolution, by controlling energy to energize heating elements of a thermal head to adjust heat energy applied to inks and controlling fused areas of inks in response to image information, an image having multistage gradations is recorded on a micro-porous paper in which a micro-porous layer, each pore having a diameter of 2 to 10 .mu.m, is formed on the surface.
However, in the conventional thermal transfer printer, when an image is recorded by using the above-mentioned wax-based ink, since the ink itself is soft, if a recorded image is rubbed, then the recorded image is deteriorated.
Moreover, when the micro-porous paper is in use, the ink is permeated into the micro-porous layer so that a clear image cannot be obtained due to the influence of the surface characteristic of the micro-porous paper.
Moreover, it has been proposed that an image is recorded by using an ink ribbon having a resin-based single-layer ink layer. When an image is recorded by using this resin-based ink, although a problem in which a recorded image is deteriorated by rubbing can be solved and a clear image having an excellent fastness can be obtained, the resin-based ink is poor in transfer sensitivity as compared with the wax-based ink so that an accurate transfer cannot be obtained particularly in the low-density portion. As a result, a jaggy is produced in the recorded image due to a transfer failure or the like. There is then the problem that a clear recorded image cannot be obtained.